The present invention relates to a method of reducing the vertical smears to be generated in a solid state image sensor of the type including a plurality of photoelectric conversion elements arrayed regularly in horizontal and vertical directions, a plurality of vertical signal lines for transferring signal charges in a vertical direction, and at least one charge transfer device for transferring the signal charges in a horizontal direction, and a solid state imaging device for practicing the vertical smear reducing method.
One example of the solid state image sensor of this type is disclosed in European patent application Publication No. 0,054,244. The equivalent U.S. patent is U.S. Pat. No. 4,443,818.
The structure of the solid state image sensor of this type is shown in FIG. 1. With reference to this Figure, the operations of the solid state image sensor of this type will also be described.
The output pulses, which are consecutively sent to the output lines 2.sub.1, 2.sub.2, - - -, and 2.sub.m of a vertical shift register 1, are added to an interlace circuit 3 so that output pulses are generated at output lines 4.sub.1, 4.sub.3, - - -, and 4.sub.2m-1 in an odd field and at output lines 4.sub.2, 4.sub.4, - - -, and 4.sub.2m in an even field. As a result, during a first horizontal scanning period of the odd field, vertical switching transistors 5.sub.1-1, 5.sub.1=2, - - -, and 5.sub.1-n are simultaneously turned on so that the signal charges generated in photo diodes 6.sub.1-1, 6.sub.1-2, - - -, and 6.sub.1-n are transferred to vertical signal lines 7.sub.1, 7.sub.2, - - -, and 7.sub.n corresponding thereto, respectively. Likewise, during a first horizontal scanning period of the even field, vertical switching transistors 5.sub.2-2, 5.sub.2-2, - - -, and 5.sub.2-n are simultaneously turned on so that the signal charges generated in photo diodes 6.sub.2-1,6.sub.2-2, - - -, and 6.sub.2-n are transferred to the vertical signal lines 7.sub.1, 7.sub.2, - - -, and 7.sub.n. By the ON signal from a control terminal 9, moreover, horizontal switching transistors 8.sub.1, 8.sub.2, - - -, and 8.sub.n are simultaneously turned on so that the signal charges of the vertical signal lines 7.sub.1, 7.sub.2, - - -, and 7.sub.n are transferred to a charge transfer device 10 (which will be shortly referred to as "CTD"). This CTD 10 acts as a horizontal shift register.
Here, a portion having photosensitivity is desired to be limited to a portion of a photo diode 6. As a matter of fact, however, the peripheral portion of the photo diode 6, e.g., the drain of a vertical switching transistor (e.g., a MOS transistor) 5 may have the photosensitivity. Those charges generated in the drain of the transistor 5 are transferred to the vertical signal lines 7 irrespective of the conductivity of the vertical switching transistors 5. With the respective vertical signal lines 7, however, there are connected all the drains of hundreds of vertical switching transistors 5 arrayed in the vertical direction so that the charges generated in all the drains are mixed and added until they are stored in the vertical signal lines 7. The charges thus stored are obtained such that they are superimposed on the normal signal charges generated in the photo diodes 6 for the respective horizontal scanning periods. As a result, if an object having such a bright portion as is shown in FIG. 2A, for example, is imaged, a vertical smear of the bright portion appears in a reproduced frame, as shown in FIG. 2B. This noise component intrinsic to the solid state image sensor is called the "vertical smear."
The magnitude of the charges resulting in the vertical smear is proportional to their storage period in the vertical signal lines. When the horizontal switching transistors 8 are turned on, more specifically, the magnitude of the vertical smear charges to be transferred to the horizontal CTD is proportional to the period from the instant when the horizontal switching transistors 8 are previously turned off.
With this in mind, sweeping out transistors 11.sub.1, 11.sub.2, - - -, and 11.sub.n equipped with a common control terminal 13 are connected in parallel with the horizontal switching transistors 8.sub.1, 8.sub.2, - - -, and 8.sub.n such that they are turned on during the horizontal blanking period.
FIG. 3 is a time chart showing the driving pulses of FIG. 1. During the period t.sub.a -t.sub.b in the horizontal blanking period as shown in FIG. 3, a sweeping out transistor 11 is first supplied with a driving pulse to sweep out from a sweeping out terminal 12 to the outside the vertical smear charges Q.sub.V1 which are stored on and after the ON period (t.sub.e '-t.sub.f ') before the one horizontal scanning period. Subsequently, during a period t.sub.c -t.sub.d, the vertical switching transistors 5 are turned on to transfer the signal charges Q.sub.S of the photo diodes 6 to the vertical signal lines 7, and during the period t.sub.e -t.sub.f the horizontal switching transistors 8 are turned on to transfer the signal charges Q.sub.S to the horizontal CTD. At this time, into these signal charges, there are mixed the vertical smear charges Q.sub.V2 which are stored in the period t.sub.b -t.sub.f.
As a result of the transfer during the horizontal scanning period, the charges obtained from the output of the horizontal CTD 10 are converted by the gate capacitor C.sub.G of a source follower transistor 14 into such a voltage signal V.sub.out as is expressed by the following Equation until it appears at an output terminal 15: ##EQU1##
Here, if the one horizontal scanning period T.sub.0 and the period t.sub.b -t.sub.f are designated by T.sub.0 and T.sub.1, respectively, the ratio a between the vertical smear charges Q.sub.V0 (=Q.sub.V1 +Q.sub.V2) without the sweeping out operation and the vertical smear charges Q.sub.V2 with the sweeping out operation is expressed by the following Equation: EQU a=Q.sub.V2 /Q.sub.V0 =T.sub.1 /T.sub.0 ( 2)
Since the one horizontal scanning period and the horizontal blanking period are about 64 .mu.S and 11 .mu.S, respectively, the ratio a is smaller than 1/6. Thus, a remarkable improvement can be obtained by conducting the sweeping out operation. In order to prevent the generation of the vertical smears for all the objects, however, that ratio has to be reduced to about 1/100.
In the aforementioned European Patent Application Publication No. 0,054,244, there is disclosed the steps of: transferring the noise charges, which are stored in vertical signal lines, to a horizontal CTD prior to the reading out of signal charges from photoelectric conversion conversion elements such as photo diodes located on horizontal lines a and b; reading out the signal charges and the noise charges from the horizontal CTD; and eliminating the noise charges mixed in the signal charges by subtracting the noise charges from the signal charges.
Despite this fact, however, the European Patent Application merely discloses the above-specified principle but does not disclose a method of accurately eliminating noise components from a signal.